Effectiveness of Sunscreens Against Ultraviolet Radiation using Bacteria
Name _____________________________________
Partner _________
Introduction
Enough exposure to UV light is lethal to bacteria. UV light may also cause
damage to human skin cells. It is thought this damage may destroy the ability of the skin
cells to control cell division, thus leading to skin cancer. For this reason people are
encouraged to protect their skin from potentially harmful effects of UV light by using
sunscreens. In theory, higher SPF values give greater protection from UV light.
Purpose
To determine blocking ability of sunscreens
Materials per group of 4 students
Four nutrient agar plates (two for each
sunscreen)
Plastic wrap
Aluminum foil
UV light
Sterile cotton applicators or pipettes /
bacteria spreaders
24 hour broth culture of Serratia
marcescens
Sunscreens
Analytic balances
Masking tape
Procedure – DAY 1
1. Label the bottom of your four Petri dishes with your group name and date. Divide
the plate in half by drawing a line on the bottom half of the plate (part with agar).
Label one side “C” for control and the other side with sunscreen and SPF.
2. While holding the lid to avoid contamination, pipette 0.1 mL of bacteria from the
sample into each dish. Using the bacteria spreader, spread the sample evenly
throughout the entire dish. Or using a sterile cotton applicator, streak innoculate a
nutrient agar plate to form a bacterial lawn over the entire surface. Repeat this
process for all remaining plates.
3. Cut plastic wrap large enough to cover the top of each Petri dish. Remove the lid
and cover the top of the Petri dish with the plastic wrap. Snuggly fold the edges
beneath the bottom of the Petri dish. Secure the edges of the plastic wrap with
masking tape. Keep the Petri dish lids for later use.
4. Weigh out 0.5 grams of sunscreen for each Petri dish. Apply the sunscreen
evenly to the plastic wrap using the same technique for each sunscreen.
5. Cut and fold in place pieces of aluminum foil large enough to cover the control
half of each Petri dish.
6. Using your UV light source (can be sunlight), expose your Petri dishes to the light
for three minutes making sure dishes are same distance from light source.
7. Once samples have been exposed to UV light, remove the aluminum foil and
plastic wrap. Put the Petri dish lids back on.
8. Stack your plates into groups of four and tape them together with the masking
tape. Make sure your group name is clearly written on the masking tape. Place
the dishes, inverted, into the incubators (250 C)
Procedure – DAY 2
1. Following 24-48 hours growth, determine the number of bacterial colonies
growing on your Petri dishes.
2. The number of colonies represents the bacteria that survived the UV light and
reflects the amount of protection the sunscreen offered.
3. Accurately do a colony count on the control sides of the two Petri dishes for your
first sunscreen. These numbers represent your raw data.
4. Add these numbers together and divide by two. This number represents your
averaged data for that control.
5. Repeat the colony count on the two plates containing the second sunscreen tested.
6. Add these numbers together and divide by two to obtain an average.
7. Now perform colony counts on the experimental sides of the two plates for your
two experimental groups. Record the raw data and average for each group.
8. Complete Data Table 1
9. Make a bar graph comparing the percent survival of your two experimental
groups.
10. Calculate the % survival for each experimental group:
Percent survival = Average colony count with sunscreen (experimental)
Average colony count with aluminum foil (control)
11. Dispose of bacteria plates properly.
Data Table 1
Sunscreen
Colonies (Control)
Colonies (Experimental)
Questions
1. What is the % survival for your sunscreens? How does this compare with your
expectations?
2.
Can we use bacteria as a model to explain the effects of UV light on human skin
cells? Suggest further experiments that may help clarify your results.